1. Field of the Invention
The present invention pertains to a quantum well semiconductor laser.
2. Description of the Prior Art
The principle of semiconductor lasers is well known. The most commonly used structure is that of the double heterostructure laser. The structure, refraction index and energy levels of this type of laser are shown in FIG. 1. Through the optical confinement induced by the difference in index among the materials 1, 2 and 3 (n1 to n3 respectively), the optic wave of the laser is confined in a space that is smaller than the natural extension of the wave in a homogeneous medium. Through the discontinuities of the energy levels in the conduction band (E.sub.c1 to E.sub.c3) and the valence band (E.sub.v1 to E.sub.v3) the electrons and the holes are concentrated in the layer 2, which acts as the active medium of the laser in giving gain to the optic wave by induced recombination of the electron-hole pairs.
Quantum well lasers have recently been introduced, wherein the active layers 2 are narrow enough to give rise to quantum-related phenomena. It has been extensively demonstrated that, in appropriate applications, these lasers perform better than double heterostructure lasers. Four different embodiments of these quantum well lasers have been shown in FIGS. 2a-2d.
A detailed modelling of such lasers shows that two parameters play an essential role in limiting the performance characteristics of these lasers: the quantum wells .DELTA.E.sub.c and .DELTA.E.sub.v are not deep enough. The separations of energy between quantum levels n =1 and n =2 are not big enough, especially in the valence band, for the holes.
The latter parameter can be improved by using very thin wells. This would increase the difference between quantum levels. However, in this case, the levels all approach the upper edge of the quantum wells. This leads to an overflowing from these wells, when they are filled with electrons and holes by injection, and this overflowing makes them in fact useless.
The normal approach would use a material, for the wells of the semiconductor components, that has a very small forbidden band, hence with great energy band discontinuities .DELTA.E.sub.c and .DELTA.E.sub.v, thus enabling the use of very thin layers without reaching the state of overflow described above. However, this method is impracticable for metallurgical reasons: materials that have big energy band differences between them generally have unequal crystal meshes. Under these conditions, when it is sought to make them grow by epitaxy, a great many dislocations are formed as soon as the thickness reaches what is known as a critical value. These numerous dislocations make the material unsuited to the operation of the laser.
An object of the present invention is a quantum well semiconductor laser that performs more efficiently than prior art lasers and is easy to make.